Such contactless measuring devices are used in a number of branches of industry, e.g. in the processing, chemicals, and foods industries.
For fill level measurement, short transmission signals, e.g., microwaves or ultrasonic waves, are periodically sent to the surface of a fill substance by means of a transmitting and receiving element, followed by receipt of their echo signals reflected on the surface, after a distance-dependent travel-time. An echo function is formed representing the echo amplitudes As a function of travel-time. Each value of this echo function corresponds to the amplitude of an echo reflected at a certain distance from the antenna.
From the echo function, a wanted echo is determined which corresponds probably to the reflection of the transmitted signal on the surface of the fill substance. In such case, it is as a rule assumed that the wanted echo exhibits a greater amplitude than the remaining echoes. From the travel-time of the wanted echo, in the case of known propagation velocity of the transmission signals, the distance between the fill substance surface and the antenna directly results.
Usually it is not a received raw signal which is used for the evaluation, but, instead, its so called envelope curve. The envelope curve is produced by rectifying and filtering the raw signal. For exact determination of a travel-time of the wanted echo, first a maximum of the envelope curve is determined.
This conventional manner of proceeding operates faultlessly in a large number of applications. Problems do, however, arise always in those cases in which it is not possible to unequivocally identify the echo coming from the fill substance. This can, for example, be the case, when installed structures are present in the container and these installed structures reflect the transmission signals better than the fill substance surface.
In such cases, it is possible, e.g. at startup, to specify once for the fill level measuring device the current fill level. The fill level measuring device can identify the associated echo as wanted echo on the basis of the specified fill level and e.g. follow such by a suitable algorithm. In such case, e.g. in each measuring cycle, maxima of the echo signal or echo function are determined, and, on the basis of knowledge of the fill level determined in the previous measuring cycle and an application-specific maximally expected rate of change of the fill level, the wanted echo is determined. From the travel-time of the so determined current wanted echo, the new fill level is then determined.
Starting with the position of the wanted echo in the previous measuring cycle, a time window is determined in which the wanted echo of the current measuring cycle must be. The current wanted echo can, however, only then be found in this time window when the wanted echo of the previous measuring cycle could be determined and, in the current measuring cycle, a maximum corresponding to the current wanted echo can be found.
The travel-time of the maximum of the wanted echo is, however, an echo property which cannot always be determined. For instance, if the fill level is located in the vicinity of a fixedly installed disturbance, e.g. a holder in the interior of the container, then the echoes of the disturbance and the fill substance are superimposed. An identification of the wanted echo is then not always possible. Similar problems arise when elements extend into the signal path only sporadically, such as is the case e.g. with stirrers which suddenly emerge in the vicinity of the fill substance surface in the signal path and reflect the transmission signals, or when the reflection characteristics of the fill substance changes, e.g. due to foam formation on the surface.